Disk Size Calculator
Enter the capacity printed on the box, the file system and the space taken by the system and recovery partitions. The calculator shows the size your computer reports, why it differs, and how much space is free for your files.
- Shown capacity
- Free space
- TB against TiB
Windows uses binary units with decimal names. macOS shows the box size.
Disk makers count in thousands, computers in 1,024s. A 1 TB disk holds 1,000,000,000,000 bytes, which Windows shows as about 931 GB. Every terabyte disk looks about 9% smaller than the box says. Nothing is missing, it is just counted in different units.
Advertised and shown sizes
Windows view. Search for a size.
| Box | Bytes | Windows shows |
|---|---|---|
| 128 GB | 128,000,000,000 | 119 GB |
| 256 GB | 256,000,000,000 | 238 GB |
| 500 GB | 500,000,000,000 | 465 GB |
| 1 TB | 1,000,000,000,000 | 931 GB |
| 2 TB | 2,000,000,000,000 | 1.82 TB |
| 4 TB | 4,000,000,000,000 | 3.64 TB |
| 8 TB | 8,000,000,000,000 | 7.28 TB |
| 16 TB | 16,000,000,000,000 | 14.55 TB |
Before file system overhead.
Box size and shown size
Windows, NTFS. Slide the disk size.
The bars compare the box size with what the computer shows.
How to work out shown capacity
Box to screen. Step through it.
Box size
In bytes.
The operating system and updates can take 30 GB to 60 GB on a system disk.
NTFS, APFS or exFAT
File systems. Compare them.
Windows.
Mac.
Portable.
Linux ext4 reserves 5% for the system by default, which can be reduced on data disks.
Before you buy
Disk checks. Tick each item.
Tick each item as you go.
Keep about 10% to 20% free on solid-state disks for speed and long life.
What you get
Disk capacity. Each tile explains one result.
Size.
Exact.
Per cent.
GB.
For files.
1 TiB is 1,099,511,627,776 bytes, almost 10% more than 1 TB.
Frequently Asked Questions
No question matches that search.
Why does my one terabyte drive show nine hundred and thirty-one gigabytes?
The maker counts a terabyte as a trillion bytes. Windows divides by one thousand and twenty-four three times.
Is any space actually missing?
No. The same bytes are just counted with a different unit.
How much space does the file system take?
Usually well under one per cent, except ext4, which reserves five per cent.
Why does a Mac show the full size?
macOS counts in decimal units, like disk makers.
How much free space should I keep?
About ten to twenty per cent, especially on solid-state disks.
What is a TiB?
A tebibyte, two to the power forty bytes.
How much space does Windows take?
Roughly thirty to sixty gigabytes, including updates.
No more missing gigabytes
Know what your computer will show before you buy, and plan your space with real numbers.
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